This invention relates to wet friction materials which run in a liquid lubricant and are employed in clutch, brake, transmission assemblies and the like.
Conventional wet friction materials are normally prepared by impregnating a wet-laid paper of cellulose or other fibers with a thermosetting resin and possible other friction modification agents or particles. The resin is cured by application of heat and pressure. The treated paper is adhered to a metal backing plate, and grooves may be formed on the surface of the friction layer to improve performance and circulation of cooling lubricant. The paper is made by conventional papermaking methods wherein a dilute slurry of fibers and other possible additives are deposited on a porous conveyor and dried.
Wet friction materials are formulated with ingredients and processes to achieve the frictional performance required for the desired or particular application. Significant performance indices include coefficient of friction, wear, material compression, fatigue, friction stability and torsional response. As discussed above, the majority of the current wet friction material are made using a wet-laid fiber process which provides a porous matrix of fibers, filler, binders and friction particles as required to achieve the desired performance.
There is a continuing need to develop and provide wet friction materials having improved performance under harsh conditions, such as may be encountered in continuously slipping torque converters, active transfer cases, slipping differentials and other high termally loaded applications.
Several proposals have been made to use carbon fibers or powders in wet friction materials. For example, it is known to employ fabrics of woven carbon yarns, or sprinkled carbon particles and pyrolytic carbon, in order to improve performance and durability at high temperatures.
Wet friction materials are easily distinguished from so-called dry friction materials which are designed to run in air rather than in a lubricating fluid. Dry friction materials have low porosity due to the high level of binders or resins, with the binder content greatly in excess of 80% by weight and the material being subjected to high densification. Wet friction materials, on the other hand, are only partially filled with cured resin and are designed to have good porosity, allowing penetration of the liquid lubricant/cooling liquid.
In accordance with the present invention, a continuous length of yarn is wound into a flat wafer shape or flat ring to provide the substrate for a wet friction material. The yarn comprises fibers or filaments of a high temperature and wear resistant natural or synthetic material, such as carbon, aramid, glass and the like, as well as mixtures of the former with each other or other types of fibers or filaments. The yarn or wafer is coated or impregnated with a suitable thermosetting resin, typically less than 80% by weight of the wafer, in order to allow a degree of porosity to be retained. The resin may contain conventional friction modifying particles and other agents. The resin impregnated wafer may then be cured and bonded to a metal backing disk or core or other relatively rigid support. In the alternative, an impregnated wafer may be formed and bonded to a core in a one step operation by forming the wafer directly onto the core.
In a preferred embodiment, the yarn is formed on a flat support or supports in a continuous spiral fashion, preferably with no overlapping of the yarn. This type of winding is referred to herein as xe2x80x9ccircumferential woundxe2x80x9d, and the yarns in the wafer may be closely adjacent or in contact. Overlapping yarn patterns are also possible to produce other versions of nonwoven fabrics but are less desired.
The wet friction material of the present invention is characterized in that it is not woven or braided but comprises a continuous length of yarn, without the yarn having attachment points except by virtue of the cured resin impregnated therein.
In an additional embodiment, the wound friction material is supported by a sublayer or backing of a layer of material which is more compliant than the friction material layer. This sublayer may take the form of a porous paper or other nonwoven bonded lightly with a thermosetting resin. The resulting friction plate would therefore comprise a metal backing plate or core, with splines on the inner or outer circumference, together with the outer layer of bonded nonwoven friction material with a sublayer of a compliant material bonded to one or both sides of the core.
The winding of the yarn is preferably carried out by winding between a pair of separable spaced flat platens having a central cylindrical core and being spaced apart approximately equal to the diameter of the yarn, together with a possible sublayer of compliant material.
The nature of the nonwoven porous friction material substrates of the present invention is adaptable to many variations in order to suit a variety of end use applications and properties. The yarn itself may comprise a blend of two or more different fibers and/or filaments. In addition, two or more stacked layers of wafers may be employed.